Imaging devices, such as scanners, copiers, facsimile machines, and multipurpose machines, are commercially available for converting tangible images (e.g., documents, photographs) into electronic images. These imaging devices operate by generating light from a light source in the imaging device and reflecting this light off of the surface of the tangible image. The reflected light is collected on an array of photosensitive elements (e.g., a charge coupled device (CCD)), which convert the reflected light into electronic signals representing the image.
More recently, imaging devices have been proposed that use a digital camera and an LED lamp. The camera is aimed at a document and the LED lamp illuminates the document for the camera. In order for the LED lamp to provide adequate light, as well as prevent stray light leaking out across the room, the LED is focused onto the platen. The focusing transforms the LED's Lambertian radiance into a cone. Low cost focusing mechanisms such as Fresnel lenses or back-reflectors form non-uniform cones. The non-uniformity can reach 50% and severely degrade image quality.
The focused light reflected off a document is both diffuse and specular. The diffuse hotspot causes lighter regions of captured documents to appear dirty and grey colored. The specular hotspot causes darker regions to be speckled and washed-out. Camera manufacturers typically calibrate the white and black non-uniformities of a sensor array. This “zonal correction” calibrates offset, gain, sensor shading, and optical lens falloff, but does not calibrate for illumination. Thus, in previous attempts to accommodate camera-based scanners, the user often becomes frustrated when the resulting scanned image does not meet their expectations.